I always calibrate to the point that a 90gsm sheet of paper is tight between the nozzles and the bed. ie: there is a fair bit of friction.

Sounds like you have too large a gap.

bear in mind that once the raft is printed the print head then leaves a small gap between the filament and the surface of the raft. So if you're fairly loosely calibrated to begin with, this gap will become significant and probably cause issues.

Also if you're using pla - why are you using a raft ?
Totally unnecessary. just slaver a fe wlayers of gluestick pva on yhe bed, calibrate tightly and you only issue will be getting the model off the bed ( do it while the buildplate is hot)